This invention pertains to the repair of parts comprising metals, and surfaces and coatings of said parts using reactive metals coating processes. Coating and surface repair fall under U.S. Patent Class 427 (COATING PROCESSES), Subclass 140 (Processes directed to the restoration or repair of coatings or surfaces of objects). Surface treatments via reactive metal coating processes fall under U.S. Patent Class 148 (METAL TREATMENT), Class Definition C ( . . . processes of reactive coating of metal wherein an externally supplied carburizing or nitriding agent is combined with the metal substrate to produce a carburized or nitridized or carbonitrided coating thereon or a uniformly carburized, nitrided, or carbonitrided metal alloy containing a metal element from said substrate) and Class Definition D ( . . . processes of reactive coating of metal wherein an externally supplied agent combines with the metal substrate to produce a coating thereon which contains at least one element from said metal substrate). This invention is applicable in maintenance and restoration of parts in many industries including, but not limited to, aviation and space industries.
Various processes are well-known for providing coatings or modified surfaces on metals to protect them from effects such as wear, erosion, and corrosion. Such processes include chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spray, and reactive coating (boronizing, carburizing, nitridizing, carbonitridizing, etc.). For instance, U.S. Pat. No. 5,272,014 (Leyendecker) teaches a wear-resistant CVD coating for substrates such as forming or cutting tools. U.S. Pat. No. 5,656,364 (Rickerby) and U.S. Pat. No. 5,702,829 (Paidassi) teach multiple-layer erosion-resistant PVD coatings for substrates such as gas turbine engine compressor or turbine blades. U.S. Pat. No. 4,850,794 (Reynolds, Jr.) teaches solution-bath and gas nitriding to enhance the wear-resistance of steam turbine components. U.S. Pat. No. 4,588,450 (Purohit) teaches nitriding of nickel-based super alloys including inconel to improve their creep strength, fatigue strength, and resistance to oxidation. U.S. Pat. No. 6,129,988 (Vance , et al.) teaches gas nitriding of metallic bond coatings for thermal barrier coating systems. Nitriding of metallic bond coatings enhances oxidation resistance thereby prolonging the adherence of ceramic thermal barrier coatings applied thereon. CVD, PVD and plasma spray processes generally involve deposition of additional material on the surface of a substrate. Reactive coating processes generally involve incorporation or dispersion of additional chemical constituents into the existing lattice structure of a metal substrate.
Reactive coating processes are known for producing treated surfaces with chemical compositions that vary as a function of depth, also known as functionally gradient surfaces. For instance, surfaces produced via nitriding consist of a hard nitride layer above a nitrogen-containing diffusion zone, with nitrogen content gradually decreasing deeper into the substrate material. Richter discusses a plasma nitriding process for producing functionally gradient surfaces on stainless steel and aluminum alloys (xe2x80x9cNitriding of Stainless Steel and Aluminum Alloys by Plasma Immersion Ion Implantationxe2x80x9d, Surface and Coatings Technology, Vol. 128-129, 2000, pp. 21-27). U.S. Pat. No. 4,762,756 (Bergmann) teaches a plasma nitriding process that is enhanced using arc discharge, whereby functionally gradient surfaces are produced on metals including stainless steel and titanium. Meletis discusses an enhanced plasma nitriding process for producing functionally gradient surfaces on titanium (xe2x80x9cCharacteristics of DLC Films and Duplex Plasma Nitriding/DLC Coating Treatmentsxe2x80x9d, Surface and Coatings Technology, Vol. 73, 1995, pp. 39-45). This (enhanced nitriding process is also taught in expired U.S. Pat. No. 4,460,415 (Korhonen, issued Jul. 17, 1984) and U.S. Pat. No. 5,334,264 (Meletis, issued Aug. 2, 1994). U.S. Pat. No. 4,568,396 (Vardiman) teaches a carburizing method via carbon ion implantation wherein carbon content of the treated surface varies as a function of depth. PVD and CVD processes are better-known for producing coatings of uniform composition as a function of depth (monolayers), but can also be adapted to produce functionally gradient surfaces. For example, U.S. Pat. No. 5,989,397 (Laube) teaches a method and apparatus for producing deposited surfaces with depth-varying compositions of titanium, carbon, and nitrogen.
A review of enhanced nitriding processes is presented by Czerwiec et al (xe2x80x9cLow-pressure, high-density plasma nitriding: mechanisms, technology and resultsxe2x80x9d, Surface and Coatings Technology, Vol. 108-109, 1998, pp. 182-190). These processes can be classified under the following four categories: Thermionically assisted d.c. triode (TAT); plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII); electron cyclotron resonance (ECR) systems; and thermionic arc discharge (TAD). A version of the TAT enhanced plasma nitriding method and apparatus presented by Meletis in U.S.
Pat. No. 5,334,264 is previously taught by expired U.S. Pat. No. 4,460,415 (Korhonen), and also by earlier references including Matthews and Teer (xe2x80x9cCharacteristics of a Thermionically Assisted Triode Ion-Plating Systemxe2x80x9d, Thin Solid Films, Vol. 80, 1981, pp. 41-48), Korhonen and Sirvio (xe2x80x9cA New Low Pressure Plasma Nitriding Methodxe2x80x9d, Thin Solid Films, Vol. 96, 1982, pp. 103-108), Korhonen et al (xe2x80x9cPlasma Nitriding and Ion Plating With an Intensified Glow Dischargexe2x80x9d, Thin Solid Films, Vol. 107, 1983, pp. 387-394), Fancey and Matthews (xe2x80x9cSome Fundamental Aspects of Glow Discharges in Plasma-Assisted Processesxe2x80x9d, Surface and Coatings Technology, Vol. 33, 1987, pp. 17-29), Ahmed (xe2x80x9cIon Plating Technology, Develoments and Applicationsxe2x80x9d, John Wiley and Sons, New York, 1987, pp. 68-70), Fancy and Matthews (xe2x80x9cProcess Effects in Ion Platingxe2x80x9d, Vacuum, Vol. 41, No. 7-9, 1990, pp. 2196-2200), and Leyland et al (xe2x80x9cEnhanced Plasma Nitriding at Low Pressures: A Comparative Study of D. C. and R. F. Techniquesxe2x80x9d, Surface and Coatings Technology, Vol. 41, 1990, pp. 295-304. Furthermore, Molarius et al teaches that the process of U.S. Pat. No. 4,460,415 (Korhonen) can be used to treat titanium (xe2x80x9cIon Nitriding of Steel and Titanium at Low Pressuresxe2x80x9d, 4th Int. Congress on Heat Treatment of Materials. Jun. 3-7, 1985. Berlin (West), Proceedings, Vol I, p. 625-643. Hxc3xa4rterei-Technische Mitteilungen 4(1986)6, 391-398.). These references establish prior art that pre-dates the filing of the Meletis Patent by 2 to 10 years. None of these references is cited in the Meletis Patent. U.S. Pat. No. 5,334,264 therefore teaches very little that was not previously taught by prior art.
Functionally gradient surfaces are known to have superior wear and erosion properties compared to monolayer coatings. Voevodin presents results of scratch tests for multiple-layer titanium, titanium carbide, and diamond-like carbon (DLC) surfaces prepared using the process of U.S. Pat. No. 5,989,397 (xe2x80x9cDesign of a Ti/TiC/DLC Functionally Gradient Coating Based on Studies of Structural Transitions in Ti-C Thin Filmsxe2x80x9d, Thin Solid Films, Vol. 298, 1997, pp. 107-115). Meletis presents results of wear tests for functionally gradient, nitrided titanium surfaces (xe2x80x9cCharacteristics of DLC Films and Duplex Plasma Nitriding/DLC Coating Treatmentsxe2x80x9d, Surface and Coatings Technology, Vol. 73, 1995, pp. 39-45). Gachon presents results of erosion tests for functionally gradient, multiple-layer tungsten carbide coatings (xe2x80x9cStudy of Sand Particle Erosion of Magnetron Sputtered Multilayer Coatingsxe2x80x9d, Wear, Vol. 233-235, 1999, pp. 263-274). Gupta presents results showing that PVD multilayer titanium nitride coatings have superior erosion resistance compared to titanium nitride monolayer coatings on turbine engine compressor blades (xe2x80x9cProtective Coatings in the Gas Turbine Enginexe2x80x9d, Surface and Coatings Technology, Vol. 68/69, 1994, pp. 1-9). Because of their superior performance, functionally graded surfaces are preferred over monolayer coatings. In general, thicker coatings or surface treatments (monolayer or functionally gradient) tend to provide better wear and erosion protection.
Coating or surface treatment thickness determines not only wear and erosion resistance, but can also affect fatigue strength of the substrate. For instance, previous attempts to plasma nitride titanium and titanium alloys have most often produced surfaces with increased wear resistance, but often reductions in substrate fatigue strength. Morita presents a list of references dating from 1964 to 1996 for which this is true (xe2x80x9cFactors Controlling the Fatigue Strength of Nitrided Titaniumxe2x80x9d, Fatigue and Fracture of Engineering Materials and Structures, Vol. 20, No. 1, 1997, pp. 85-92). Morita also shows the relationship between substrate fatigue strength, substrate grain size, and surface treatment depth (case depth) for nitrided titanium. Morita gas nitrided samples at temperatures from 620 degrees C. to 1200 degrees C. to achieve a range of case depths and grain sizes. Results show that for equivalent grain sizes, the fatigue strength of nitrided titanium with a case depth of 40 micrometers is greater than the fatigue strength of the untreated substrate. When the case depth is increased to 100 micrometers (same grain size), fatigue strength of the nitrided material is significantly decreased compared to the untreated substrate. These results apply over a wide range of grain sizes. The diffusion zone of the nitrided surface appears to help suppress crack propagation in the substrate, but only to a limited degree. The tendency of the 40 micrometer depth case to fracture and initiate substrate crack growth tends to be countered by decreased tendency for slip and dislocations in the diffusion zone. Under a similar level of substrate strain the 100 micrometer case is more likely to fracture, and the diffusion zone is unable to counter the increased tendency for crack growth. Morita""s results also indicate that long nitriding times at high temperatures tend to degrade fatigue strength via excessive case thickness and excessive grain growth (e.g., material annealing).
Degradation of fatigue strength due to thick coatings on turbine engine compressor blades is mentioned by Friedrich (xe2x80x9cImproving Turbine Engine Compressor Performance Retention Through Airfoil Coatingsxe2x80x9d, NASA Lewis Research Center Aircraft Engine Diagnostics, Document ID 19810022661 N (81N31203), January 1981, pp. 109-117) and in U.S. Pat. No. 4,761,346 (Naik). There appears to be a correlation between thick coatings and degradation in fatigue strength. Thicker coatings tend to provide better wear and erosion protection but often at the expense of fatigue strength. These factors must be considered carefully for coatings and surface treatments, particularly in applications where superior fatigue strength is important.
Despite improved protection of the substrate, monolayer coatings or functionally gradient surfaces will eventually wear, erode, or corrode in-service and the underlying metal substrate can be exposed. In general, damage to coated or treated surfaces is not uniform, and consists of local damage sites surrounded by areas where the coating or surface treatment is intact. This is particularly true in cases where the surface has experienced impact or micro-chipping damage due to erosive service conditions. For instance, Gupta shows localized damage to a titanium nitride coated turbine engine compressor blade (xe2x80x9cProtective Coatings in the Gas Turbine Enginexe2x80x9d, Surface and Coatings Technology, Vol. 68/69. 1994, pp. 1-9). Once damaged, coated or treated parts must be restored or repaired to reestablish the original level of protection provided to the substrate.
Damaged areas of some coatings can be cleaned of loose debris and the surface spot-repaired or re-coated. For instance, U.S. Pat. No. 5,958,511 teaches a process for spot-repairing conversion coatings such as Alodine (Henkel Surface Technologies, Madison Heights, Mich.xe2x80x94formerly Parker-Amchem). U.S. Pat. No. 3,248,251 (Allen) describes aluminum-filled inorganic phosphate overlay coatings that are used to protect components in turbomachinery. A commercial version of this coating manufactured by Sermatech International Inc. (Limerick, Pa.) is reportedly spot-repairable. U.S. Pat. No 6,042,880 (Rigney) teaches repair and spot-repair of metallic bond coats used under thermal barrier coatings (TBCs) on turbine blades, wherein the TBC is completely removed to expose the bond coat, then the bond coat spot-repaired. Rigney emphasizes that complete removal of the TBC and bond coat, and simultaneous unintentional removal of substrate is detrimental to blade fatigue life.
Other more durable coatings including some produced via CVD, PVD, or plasma spray processes are not typically spot-repaired. Usual practice for these coatings is to completely remove all old surface materials, thereby helping to ensure the integrity of the replacement coatings. For instance, U.S. Pat. No. 5,368,444 (Anderson) discusses the strip and re-coat of copper-nickel-indium anti-fretting and anti-wear coatings commonly employed on compressor and (turbine blade dovetails. U.S. Pat. No. 5,813,118 (Roedl) and U.S. Pat. No. 6,049,978 (Arnold) describe grit blast and chemical stripping for turbine engine airfoils. U.S. Pat. No. 5,421,517 (Knudson) teaches a waterjet removal process for gas turbine engine components and also aircraft exterior surfaces. U.S. Pat. No. 6,036,995 (Kircher) teaches removal of the surface layer of a metallic coating by first applying a slurry of aluminum in an inorganic binder to the surface of a part coated with the coating, then heating the coated part to melt the aluminum which flows inward into the surface and reacts with the surface to form a brittle aluminide layer, and finally removing the layer via chemical or physical means. Coating removal processes such as these can be effective, but tend to be slow, equipment-intensive, or labor-intensive for removing durable coatings and are therefore expensive. A means to easily remove and/or spot-repair coatings such as these is needed in the art.
Another aspect of coating or surface treatment repair is addressing significant wear or damage that extends into the substrate material. In some applications, infrequent but severe damage events can occur that will breach protective coatings and penetrate deeply into the substrate. For instance, Gravett presents data from a field inspection campaign of foreign object damaged turbine engine compressor blades (xe2x80x9cThe Foreign Object Damage Project of the PRDA V HCF Materials and Life Methods Programxe2x80x9d, 4th National Turbine Engine High Cycle Fatigue Conference, Monterey, Calif., USA, Feb. 9, 1999). Data presented shows that the depth of foreign object damage to compressor blades can range from 0.02 inches to 0.5 inches, with an average depth of 0.06 inches. This average damage depth is much greater than a typical protective coating or treated surface.
Surface damage to such depths is unacceptable for some applications, but is acceptable for others. In the case of cutting tools, significant erosion or wear of the tool will cause parts machined by the tool to be out of tolerance and therefore unacceptable. However, in the case of turbine engines, significant wear and erosion on in-service compressor blades is commonplace. Gupta presents data showing local compressor airfoil erosion can be on the order of 10 percent of the original airfoil chord (xe2x80x9cProtective Coatings in the Gas Turbine Enginexe2x80x9d, Surface and Coatings Technology, Vol. 68/69, 1994, pp. 1-9). Schwind presents similar, but more detailed information regarding blade erosion (xe2x80x9cBlade Erosion Effects on Aircraft-Engine Compressor Performancexe2x80x9d, Department of Energy Report DOE/CS/50095-T2, 1982) In fact, special procedures have been developed to classify and repair such damage to turbine engine blades. U.S. Pat. No. 5,625,958 (DeCoursey) teaches a method to determine the service life remaining in a blade after erosion has occurred. U.S. Pat. No. 5,197,191 (Dunkman) teaches a method and apparatus to repair gouged out and damaged leading and trailing edges of gas turbine engine blades by cutting away a curved section including the damaged area and forming a blend radius along the repaired edge. Clearly, it would be advantageous to coat or surface treat parts such as turbine engine airfoils to improve their erosion resistance and durability, yet retain the ability to repair the parts as is common in the art.
The present invention discloses and teaches restoration of durable coatings or surface treatments on metal substrates and how to overcome deficiencies of the prior art.
Various embodiments of this invention disclose and teach the following methods of how to:
Restore damaged durable coatings or surface treatments on metal substrates.
Restore damaged CVD, PVD, plasma spray, and reactive coatings or surface treatments on metal substrates.
Restore damaged functionally gradient coatings or surface treatments on metal substrates.
Spot-repair damaged durable coatings or surface treatments on metal substrates.
Restore protective surfaces on the damaged areas of substrates without excessive buildup of repair material on undamaged areas.
Spot-repair durable coatings or surface treatments while allowing smoothing and blending of local part damage to acceptable conditions or dimensions prior to conducting the surface repair.
Spot-repair durable coatings or surface treatments to restore the protective surface over weld repair areas on metal substrates.
Reduce the difficulty of removing protective top-coats from metal substrates as part of surface repairs.
Reduce the difficulty of removing protective top-coats from metal substrates in conjunction with spot-repair of coatings or surface treatments that lie between the top-coats and the substrates.
Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of the preferred embodiment of the invention when taken in conjunction with the drawings and the appended claims.
All articles deriving from the methods disclosed in this invention are within the scope of this invention.